A lot of people have asked that question, here is my most recent
version of the answer. There are two implementations of TCP/IP for
standard Minix that I know about:

1.  Phillip Homburg (philip@cs.vu.nl) wrote a version of tcp/ip that
requires version 1.6.16 of Minix. It does not implement sockets and
requires a WD8003 ethernet adapter.  The package is available at
ftp.cs.vu.nl:/pub/philip/1.6.16.tcpip.  I have had only one report
from a user of this version other than the author. That report said
performance seemed good on those aspects he had tested.

2. Michael Temari (temari@temari.ae.ge.com) released version 4 of
Tnet, his implementation of tcp/ip for Minix, in summer 1993. Tnet can
be installed on Minix 1.5.10 (although extensive kernel patches must
be applied and the kernel recompiled) and supports sockets. I have
seen it on a number of ftp sites, including nic.funet.fi and
ftp.vmars.tuwien.ac.at. I have been in contact with a number of
users. One user, Kevin Duling (kduling@carl.org) added a few
improvements to version 4, which he sent me by e-mail, but I don't
believe they have been posted to any ftp sites. Another user, Mariusz
Ostrowski (s133766@ia.pw.edu.pl) posted to nic.funet.fi an ethernet
driver for Tnet that supports WD8003, 3c503, NE1000, and NE2000
ethernet adapters, and I have successfully installed Tnet with all of
these except the NE10000. I believe Mariusz is planning a new release
with some small corrections soon.

As far as I know, no-one has ported either of these packages to any
non-IBM Minix system.

There is a third implementation of TCP/IP as part of the Minix-386vmd
1.6.25.1 package, which is also available from ftc.cs.vu.nl. This of
course runs only on Intel 80386 or '486 machines.
 
There is a working version of Minix 1.5.10 with Tnet running on the
net as minix1.hampshire.edu. It is not meant to be an ftp site at this
point, but guests are welcome to log in as "guest" and to look at the
various configuration files in /usr/tnet. Performance is not great and
would probably deteriorate if several users logged on at once, but it
has been working reliably for almost six months, with no known crashes
due to software problems.

Any additions or corrections to these comments would be welcome.